Lady Scarlet expected a lot from this human pony and as she was pony play training it, she realized that it was not only weak, but it was also lazy and she felt that she had to make it work harder and be stronger. So she went out of her way to whip it and make it feel pain so that it dug deeper into its strength reserves to do what she wanted.
Lady Scarlet laid a trap for her ex boss and when he took the bait, she had fun riding him like a human horse. She loved that punishment and torture as it was painful and it was degrading. He could not do anything to her at that point and he was at her mercy. Then she told him to be good to his employees before she let him leave while crying.
Lady Scarlet had always admired horses but she did not like the expenses involved in owning them. She realized that since she had a slave, she technically had a human pony and after that, she never looked back on having her slave carry her around whenever she felt like. It was a lot of fun for her but it was a nightmare and torture for the slave who had to physically carry her and endure whipping.